Table of Results, showing the Relation of Density and Pressure of Saturated Steam.

The above table exhibits accurately the results at which we have arrived in regard to saturated steam; we have also obtained some results on the rate of expansion of superheated steam. These results are at present less complete than those upon saturated steam, as they do not range more than twenty degrees of temperature, in each case, above the maximum temperature of saturation. They appear, however, to show conclusively, that near the saturation point steam expands very irregularly, thus agreeing with what we know of other bodies in their physical relations at or near the point at which they change

their state of aggregation. Close to the saturation point we find a very high rate of expansion, but this rapidly declines as the steam superheats, and at no very great distance above it the rate of expansion nearly approximates to that of a perfect gas.

Thus, for instance, in experiment (6) where the point of maxim 'm saturation was 174°.92, between this and 180° the steam expanded at the mean rate of, whereas air would have expanded only; but on continuing the superheating, the coefficient was reduced between 180° and 200° from to, and for air the coefficient would have been, or almost exactly the same, and this rule holds good in every experiment; a high rate of expansion close to the saturation point diminishing rapidly to a close approximation to that of air,

0

315

APPENDICES.

APPENDIX I.

ON THE RESISTANCE OF BASALT TO CRUSHING.

SINCE the paper on Stone was printed (p. 129), I have had an opportunity of testing the resistance of Basalt, or Whinstone, a rock which does not find a place in the list given in that The following results were obtained:

paper.

Specimen 1.

Height 1.25 inches.

All the above specimens fractured by vertical fissures splitting up into thin prisms, wedge-shaped usually at one end. The mean crushing resistance of the above specimens is therefore

lbs.

(1.) 11,755

(2.) 12,643

Mean

(3.) 11,510

11,970-5-3437 tons per sq. inch.

The mean of three experiments on Granite gave 11,565 lbs. per sq. inch for the crushing resistance of that substance, and for Grauwacke from Penmaenmawr it was found to be 16,893 lbs. per sq. inch. The Irish Basalt is, therefore, equal in strength to the Granite, but inferior to the Grauwacke in the ratio of 1·0 to 1·4.

APPENDIX II.

MR. GRANTHAM'S VIEWS ON THE STRENGTH OF IRON SHIPS. SINCE the above Lecture was written I have had an opportunity of reading Mr. Grantham's paper on the Strength of Iron Ships, and it is gratifying to me to find that that gentleman takes precisely the same views as I have expressed above in regard to the weakness of our present construction of iron vessels. He adduces an additional reason for increasing the strength of the upper part, viz. the tendency of very long vessels to hog. "It is well known," he says, "that many iron steamboats are now employed in carrying heavy cargoes, whose length is eight times the beam, and I have frequently examined one vessel, that has sailed round the world, whose length is nine times the beam; nor is there anything in these vessels which would lead to the conclusion that such proportions are excessive. On the contrary, with improved construction, a much greater length may be ultimately attained,

especially in large vessels. The maximum of length in wooden ships has often been attained, and probably a proportion of six times the beam has seldom been exceeded, without showing unfavourable results, when heavy weights had to be carried.

"I shall best explain my views by describing the result of a calculation I lately made for the purpose of giving evidence in an important trial. The object sought was, to ascertain whether a vessel loaded as this was would rise or fall at the ends, or, in popular language, whether she would sag or hog. This ship was built of timber, with fine lines, rather light ends, and the cargo very evenly distributed; she was 225 feet long, and 42 feet beam. For the sake of the calculation the longitudinal elevation was divided into ten equal parts. The displacement of each section, the weight of the ship, and cargo also of each section, were calculated, when it was found that the ends were depressed by a force of 220 tons, and thus threw a strain on the centre equal to that weight multiplied by the leverage.

"The tendency, as above observed, is to build iron ships, especially iron steamers, much longer and finer than this vessel; it is clear that the excess of weight over displacement at the ends will increase in the same ratio, unless precautions are taken to reduce the weight there. Great attention has been paid to this subject in the timber built steamers of America; and it is found that vessels which to us appear dangerous, from the extreme height of deck-houses and machinery amidships, are plying throughout the year on their wild Atlantic coast with comparative safety.

"Now it is quite clear that a vessel should be so constructed, and, if possible, so loaded, that when in smooth water the weight should as nearly as possible correspond with the displacement of every portion."

In regard to the distribution of material in ships as now constructed, Mr. Grantham expresses himself strongly. "I think I shall best serve the cause I have so long advocated, by saying distinctly, that in my opinion a large proportion of the material now used in iron ships is worse than useles "Experience has confirmed the impression whi